The non-invasive imaging of internal body structures and organs by way of computed tomography (CT) is a widely used method in medical diagnostics. A high level of contrast can be achieved between bones and soft tissue parts. However, the contrast between the different soft tissue parts is suitable for diagnostic purposes to only a limited extent, as a result of small absorption differences.
Therefore, in order to increase the contrast between particular body structures and/or body fluids, contrast media are utilized. These contrast media contain an element which strongly absorbs X-ray radiation, in order to achieve an enhanced image contrast in relation to the surrounding tissues which have low absorption. Today, in radiological imaging using X-ray radiation, iodine-containing contrast media are used to image body fluids, organs and pathological processes. However, the absorption properties of iodine do not make the element the optimum material for contrast enhancement in X-ray diagnostics using tube voltages of >80 kV. This applies, in particular, to computed tomography, in which today tube voltages of up to 140 kV are used.
In the energy range of the X-ray radiation used, the X-ray density of contrast media increases in proportion to the atomic number of the contrast-creating element. The use of contrast media having higher atomic numbers as absorptive elements is therefore of particular interest for CT and, in addition to lanthanides (Pietsch et al., “Efficacy and safety of lanthanoids as X-ray contrast agents” in Eur J Radiol, Epub ahead of print 2009; WO 2007/051739), hafnium, rhenium, tantalum and tungsten have also been proposed (WO 97/03994; WO 97/03993). What all contrast media have in common, however, is that, despite the good safety properties such media possess, undesirable side effects can occur.
Nevertheless, the majority of contrast media currently used for X-ray diagnostics are based on iodine as the main X-ray attenuating component. Current equipment technology is optimized for this. In selecting elements for contrast media, the general rule is that the element should have the highest possible atomic number, since the X-ray absorption rises strongly with increasing atomic number. As the atomic number increases, the K-edge of the element used becomes displaced into the diagnostic energy window for X-ray radiation. At energies above the K-edge, the absorption increases suddenly, so that the design of X-ray devices is no longer based on the principle that the softest possible X-ray radiation gives the best contrast between water and the contrast medium.